This invention relates to an automatic sound volume control device for automatically controlling the sound volume generated according to environmental noise, and more particularly to an automatic sound volume control device which prevents the occurrence of an unpleasant feeling due to an abrupt change of a sound volume, and controls the generated sound volume in agreement with the human auditory sense characteristic and without being affected by the generated sound or human voices.
An automatic sound volume control device operates to increase the sound volume in proportion to environmental noise, so that a person can hear sounds from a stereo or radio satisfactorily at all times no matter how large the external noise. For instance in the case of an automobile, the noise level inside changes greatly depending on the running conditions and the running speed, which makes it difficult for the person in the cab of the auto to hear sounds from the car stereo or radio set. This difficulty can be overcome by controlling the sound volume according to the environmental noise.
In a conventional automatic sound volume control device for this purpose, the noise in the cab (hereinafter referred to as "cab noise") is detected with a microphone, so that the sound volume is controlled with an average noise level signal obtained from the cab noise thus detected.
However, the above-described conventional automatic sound volume control device is disadvantageous in the following point: As the sound volume control is carried out using the average level signal of the cab noise, if the noise level changes abruptly, the sound volume control is effected abruptly, giving an unpleasant feeling to the user. For instance, it is assumed that the noise level increases A times at the time instant t.sub.1 as shown in FIG. 1(a), and at the same time, the noise voltage jumps to AN(v) from N(v). In order to obtain the average level, a sound volume controlling control voltage Vc is obtained from a circuit having a time constant (.tau.). In this case, the control voltage Vc is: EQU Vc=N+(AN-N) (1-e.sup.-t/.tau.) (1)
Thus, the control voltage Vc changes abruptly at the time instant t.sub.1 as indicated in FIG. 1(b). If, in this case, a voltage-controlled amplifier section for carrying out sound volume control with the control voltage Vc has a characteristic of K/v, then the gain G of the voltage-controlled amplifier section is: ##EQU1## A time variable is included in the logarithmic term. Therefore, in association with the characteristic of the term (1-e.sup.-t/.tau.) the sound volume changes abruptly and unpleasantly.
In the case of the above-described conventional automatic sound volume control device, the sound volume control is carried out merely by detecting the cab noise. Therefore the device is disadvantageous in that positive feedback occurs, and the control is unstable because even a voice is detected as noise.
In addition, the conventional automatic sound volume control device involves various other problems such that, for instance, the noise level at which the sound volume control is started and the rate of sound volume control with respect to noise are fixed.